1. Field of the Invention
The present invention relates to a voice mixing apparatus and a method therefor, and more particularly to a voice conference system advantageously applicable to voice mixing for use in, for example, a voice conference system including both a client terminal compatible with wideband transmission and a client terminal incompatible with wideband transmission.
2. Description of the Background Art
In recent years, VoIP (Voice over Internet Protocol) telecommunications networks have been widely proliferating. The VoIP is not subjected to restriction on its voice bandwidth unlike the landline telephone network, which has its transmission band restricted to the frequency band from 300 Hz to 3.4 kHz, and therefore enables communications with more natural sound quality, or wideband sound quality. In order to transmit the wideband voice, wideband voice coding schemes are employed. Among these is one having its architecture scalable so as to be higher in compatibility to existing voice coding systems, as taught by Shigeaki Sasaki, et al., “Global Standard for Wideband Voice Coding, ITU-T G.711.1 (G.711 Wideband extension)”, NTT Technical Journal, May 2008.
The scalable voice coding system employs as its core coding the conventional voice data coding, e.g. voice data coding in the telephone frequency band per G.711, to add encoded data of a frequency band exceeding the telephone band to data encoded in the core to thereby produce encoded wideband voice data. Such a frequency band above the telephone band may sometimes be referred to as a wideband or higher-band region. One of the advantages of this scheme is the simplicity in voice mixer processing.
Voice mixing for multi-point communications such as a voice conference system requires to decode and re-encode voice data sent from a plurality of locations. The required decoding and re-encoding of voice data are carried out only on the legacy voice codec section that requires a relatively less amount of computation, while the wideband region is dealt with by simply duplicating encoded information from the speaker toward respective points. This scheme achieves wideband voice mixing with less amount of computation.
In a multi-point communications system that involves a client terminal based on the conventional voice coding in the telephone band and a client terminal that is compatible to the wideband voice coding, however, voice signals coming from such a terminal based on the conventional voice coding are transmitted and mixed only into the telephone band, thus failing to fully enjoy the benefit of the wideband coding. Moreover, a telecommunications system including both a client terminal compatible with the telephone band and a client terminal compatible with the wideband inherently involves a fundamental problem that a voice signal sent from the telephone band terminal is delivered in the form of voice signal in the telephone band even to the wideband terminal.